Plastic dental compositions are increasingly being used for dental restoration in the dental sector. Such plastic dental compositions usually include a matrix of organic resins and various inorganic fillers. Inorganic fillers predominantly comprise powders of glasses, (glass-) ceramics, quartz or other crystalline substances (e.g. YbF3), sol-gel materials or aerosils, which are added to the plastic composition as filling material.
The use of plastic dental compositions is intended to avoid possible harmful side-effects of amalgam and to achieve an improved aesthetic impression. Depending on the plastic dental compositions selected, they can be used for different dental restoration purposes, for example, for tooth fillings, as well as for securing parts, such as crowns, bridges and inlays, onlays etc.
The filling material per se is intended to minimize the shrinkage caused by polymerization of the resin matrix during curing. For example, if there is a strong adhesion between the tooth wall and filling, excessive polymerization shrinkage can lead to the tooth wall breaking. If the adhesion is inadequate, excessive polymerization shrinkage may result in the formation of peripheral gaps between the tooth wall and filling, which can promote secondary caries. Furthermore, certain physical and chemical demands are imposed on the fillers.
It is desirable to process the filling material to form powders that are as fine as possible. The finer the powder, the more homogenous the appearance of the filling. At the same time, the polishing properties of the filling are improved, which in addition to reducing the surface area available for attack also leads to improved resistance to abrasion and therefore to a longer-lasting filling. To enable the powders to be processed successfully, it is also desirable for the powders not to agglomerate. This undesirable effect tends to occur with filling materials produced using sol-gel processes.
Furthermore, it is advantageous if filler particles are coated or at least partially coated with functionalized silane, since this facilitates formulation of dental compositions and improves the mechanical properties.
Furthermore, the refractive index and color of the entire plastic dental composition, including fillers, should be as well matched as possible to the natural tooth material, so that it is as indistinguishable as possible from the surrounding, healthy tooth material. The grain size of the pulverized filler being as small as possible also helps to achieve this aesthetic criterion.
It is also important for the thermal expansion of the plastic dental composition in the typical range of use, i.e. usually between −30° C. and +70° C., to be matched to that of the natural tooth material in order to ensure that dental restoration measures are sufficiently able to withstand temperature changes. Excessively high stresses caused by temperature changes also can cause formation of gaps between plastic dental compositions and the surrounding tooth material, which in turn can form sites of attack for secondary caries. In general, fillers with the lowest possible coefficient of thermal expansion are used to compensate for the high thermal expansion of the resin matrix.
Good chemical resistance of the fillers with respect to acids, alkalis and water and good mechanical stability under load, such as, for example, during movement produced by chewing, can also contribute to a long service life for dental restoration measures.
Furthermore, for the treatment of patients, it is imperative that dental restoration measures can be seen in an X-ray image. Since the resin matrix itself is generally invisible in an X-ray image, the fillers must provide the required X-ray absorption. A filler of this type which provides sufficient absorption of X-radiation is described as X-ray opaque. Constituents of fillers, for example, certain components of a glass, or other substances, are generally responsible for X-ray opacity. Such substances are often referred to as X-ray opacifiers. A standard X-ray opacifier is YbF3, which can be added to the filler in crystalline, milled form.
According to International Standard DIN ISO 4049, the X-ray opacity of dental glasses or materials is quoted in relation to the X-ray absorption of aluminum as aluminum equivalent thickness (ALET). The ALET is the thickness of an aluminum sample which has the same absorption as a 2 mm-thick sample of the material to be tested. An ALET of 200% therefore means that a small glass plate having plane-parallel surfaces and a thickness of 2 mm produces the same X-ray attenuation as a small aluminum plate with a thickness of 4 mm. Analogously, an ALET of 150% means that a small glass plate having plane-parallel surfaces and a thickness of 2 mm produces the same X-ray attenuation as a small aluminum plate with a thickness of 3 mm.
Because plastic dental compositions in use are usually introduced into cavities from cartridges and then modeled in the cavities, such compositions should be at least somewhat thixotropic in the uncured state. This means that viscosity decreases when pressure is exerted, while it is dimensionally stable without the action of pressure.
Among plastic dental compositions, a distinction also should be drawn between dental cements and composites. In the case of dental cements, also known as glass ionomer cements, the chemical reaction of fillers with the resin matrix leads to curing of the dental composition and consequently the curing properties of the dental composition. Thus, their workability is influenced by the reactivity of the fillers. This often involves a setting process which is preceded by a radical surface curing, for example, under the action of UV light. Composites, also referred to as filling composites, contain by contrast fillers which are as chemically inert as possible, since their curing properties are determined by constituents of the resin matrix itself and a chemical reaction of the fillers often disrupts this.
Because glasses, due to their different compositions, represent a class of materials with a wide range of properties, they are often used as fillers for plastic dental compositions. Other applications as dental material, either in pure form or as a component of a material mixture, are also possible, for example, for inlays, onlays, facing material for crowns and bridges, material for artificial teeth or other material for prosthetic, preservative and/or preventive dental treatment. Glasses of this type used as dental material are generally referred to as dental glasses.
In addition to the dental glass properties described above, it is also desirable for this glass to be free from barium and/or barium oxide (BaO), which are classified as harmful to health, and also from lead and/or lead oxide (PbO) and from other barium and lead compounds.
In addition, it is also desirable for a component of dental glasses to be zirconium oxide (ZrO2). ZrO2 is a widely-used material in the technical fields of dentistry and optics. ZrO2 is readily biocompatible and is distinguished by its insensitivity to temperature fluctuations. It is used in a wide variety of dental supplies in the form of crowns, bridges, inlays, attachment work and implants.
Dental glasses therefore represent glasses of particularly high quality. Glasses of this type also can be used in optical applications, particularly if such applications benefit from the X-ray opacity of the glass. Since X-ray opacity means that the glass absorbs electromagnetic radiation in the region of the X-ray spectrum, corresponding glasses simultaneously act as filters for X-radiation. Sensitive electronic components can be damaged by X-radiation. In the case of electronic image sensors, for example, the passage of an X-ray quantum may damage the corresponding region of the sensor or result in an undesirable sensor signal which can be perceived, for example, as an image disturbance and/or disturbing pixels. For specific applications it is therefore necessary, or at least advantageous, to protect electronic components against X-radiation by using corresponding glasses to filter said components out from the spectrum of the incident radiation.
A number of dental glasses and optical glasses are known from the prior art.
For example, DE10063939 describes a dental glass which is used, for example, in self-curing dental cement compositions. This glass reacts with acids and releases ions which serve to build up a network to facilitate curing. This is an undesirable property, however, for composites which are cured using UV light. In addition, the total content of Y2O3, La2O3 and other lanthanoids is 30 to 70% by weight, which provides a refractive index above 1.61.
EP1547572 describes a glass filler material for epoxy systems and the production thereof. The particles described therein have a particle size of 0.1 μm up to 20 μm and comprise an inner and an outer zone which have different alkali metal concentrations and in which the alkali metal ions of the inner layer do not migrate into the outer layer during the period of use. The depletion of the outer layer takes place in a further step, after the melted glass has been milled, by adding an organic or inorganic acid which is subsequently washed out again. Glass powder produced in this way has a refractive index (nd) of 1.49 to 1.55. In order that the alkali metal ions can be leached out, the molten glass must have a low chemical resistance.
WO2007077680 describes radiation shielding glass having a high lanthanide content of at least 20 mol %. This contributes to refractive indexes (nd) of all glasses obtained in this way to be beyond the use range of aesthetic composite materials, the maximum refractive indexes (nd) of which are typically around 1.60.
US20070042894 describes a substrate for flat screens which consists of (60-80 mol %) SiO2, (5-25 mol %) Al2O3 and (2-5 mol %) elements from the group of rare earth oxides. In addition, the glass may contain up to 15 mol % of network-modifying elements selected from the following group: MgO, SrO, BaO, CaO, B2O3, Ta2O5, TiO2, ZrO2, HfO2, SnO2, P2O5, ZnO, SbO, As2O3 or SnO up to 20 mol %. The described glass is distinguished by particularly high melting temperatures in order to make it possible to achieve high process temperatures of at least 650° C. The substrate glass described herein does not contain any elements from the group of alkali metals (Li2O, Na2O, K2O, Rb2O, Cs2O) and cannot be processed in a cost-effective manner.
U.S. Pat. No. 6,816,319 describes a planar lens consisting of a flat glass substrate into which a refractive-index-increasing component diffuses, which results in a gradient of the refractive index within the lens. A non-homogeneous refractive index has a strong negative effect on the optical impression of the finished restoration.
U.S. Pat. No. 6,677,046 describes a glass ceramic used as a substrate for LCD displays. However, the material obtained must contain a beta-quartz phase which is obtained by subsequent heat treatment. Differences in the refractive index of the crystalline phase and of the residual glass phase make it impossible to produce a highly aesthetic filling.
U.S. Pat. No. 6,716,779 describes a glass substrate for interference filters having a relatively high coefficient of thermal expansion. This adapted coefficient of thermal expansion means that Al2O3 is limited to no more than 12 mol %. Particularly in B2O3-free glasses (which represent most of the examples in this reference) Al2O3 reduces the coefficient of expansion resulting in poor chemical resistance.
JP2000159540 describes certain aluminum borosilicate glass used for substrates in information storage technology. The glass described therein contains B2O3 which impairs the chemical resistance of the glass to an unacceptable extent and therefore is not suitable for dental glasses.
GB2232988 describes certain alkali-resistant glass fiber material which contains at least 8 mol % ZrO2. This high content results in poor melting properties and a tendency to form a Tyndall effect.
Features common to the glasses mentioned in the prior art are that they either (1) have a relatively high refractive index nd, and/or (2) have low weathering resistance and/or (3) are not X-ray opaque and/or (4) in addition are often difficult or expensive to produce and/or (5) contain components which are harmful to the environment and/or to health.